Diabetes mellitus is a growing epidemic affecting hundreds of millions of people worldwide. (Zimmer et al., 2001). Despite a recent explosion of new classes of hypoglycemic agents, the medical need remains largely unmet and innovative diagnostics and therapeutics are still urgently needed.
D-Glucose, with the synergistic effects of certain amino acids, is the major physiological stimulus for insulin secretion (reviewed in (Henquin 2000)). Net insulin production and glucose homeostasis, however, is regulated by a number of other molecules, including several classical neurotransmitters (Ahren 2000; Brunicardi, et al. 1995) that act directly on beta cells, and indirectly through other target tissues such as liver and skeletal muscle. Many of these molecules function as amplifying agents that have little or no effect by themselves, but enhance the signals generated by the beta cell glucose sensing apparatus (Henquin 2000). For example, during the cephalic phase of insulin release, acetylcholine (ACh) is released via islet parasympathetic innervation. Beta cells express the M3 muscarinic receptor (Duttaroy, et al. 2004) and respond to exogenous ACh with increased inositol phosphate production, which in turn facilitates sodium (Na+) ion exit and calcium ion entry. This results in augmented insulin vesicle exocytosis (Barker, at al. 2002).
The amino acid glutamate, the major excitatory neurotransmitter in the central nervous system, is present in both alpha- and beta-cells of the endocrine pancreas. Glutamate is stored in glucagon-containing granules (Hayashi, et al. 2003), and is proposed to enhance insulin secretion when it is released into the vicinity of islet cells (Storto, et al. 2006). The presence of metabotropic glutamate receptors on alpha- and beta-cells themselves suggests the presence of both autocrine and paracrine circuits within islet tissue involved in the regulation of insulin secretion (Brice, et al. 2002).
Other neurotransmitters, such as the monoamines, epinephrine and norepinephine, acting both systemically and via nerve terminals in the vicinity of islets, may act to suppress glucose stimulated insulin secretion by direct interaction with adrenoreceptors expressed (mainly the alpha 2 receptor) on pancreatic beta cells (Ahren 2000; El-Mansoury and Morgan 1998). Beta cells of the endocrine pancreas also express dopamine receptors (D2) and respond to exogenous dopamine with inhibited glucose-stimulated insulin secretion (Ahren and Lundquist 1985; Niswender, et al. 2005; Rubi, et al. 2005; Shankar, et al. 2006). Purified islet tissue is a source of monoamines, and has been shown to contain 5-hydroxytryptamine, epinephrine, norepinephrine and dopamine (Cegrell 1968; Ekholm, et al. 1971; Hansen and Hedeskov 1977; Lundquist, et al. 1989; Niswender et al. 2005; Wilson, et al. 1974).
Beta cells also have the biosynthetic apparatus to create, dispose of, and store specific neurotransmitters. For example, tyrosine hydroxylase, the enzyme responsible for catalyzing the conversion of L-tyrosine to L-3,4-dihydroxyphenylalanine (L-DOPA), a precursor of dopamine, L-DOPA decarboxylase, responsible for converting L-DOPA to dopamine (Rubi et al. 2005) and dopamine beta hydroxylase, the enzyme that catalyzes the conversion of dopamine to norepinephrine, are present in islet tissue (Borelli, et al. 2003; Iturriza and Thibault 1993). Thus, L-DOPA is rapidly converted in islet beta-cells to dopamine (Ahren, et al., 1981; Borelli, et al. 1997).
Monoamine oxidase (MAO) is a catabolic enzyme responsible for the oxidative de-amination of monoamines, such as dopamine and catecholamines, and maintains the cellular homeostasis of monoamines. The possible role of MAO in islet function has been studied, (Adeghate and Donath 1991) and MAO has been detected in both alpha- and beta-cells of pancreatic islet cells, including beta cells (Feldman and Chapman 1975a, b). Interestingly, some MAO inhibitors have been shown to antagonize glucose-induced insulin secretion (Aleyassine and Gardiner 1975). The secretory granules of pancreatic beta cells store substantial amounts of calcium, dopamine and serotonin (Ahren and Lundquist 1985).
In the central nervous system, the storage of monoamine neurotransmitters in secretory organelles is mediated by a vesicular amine transporter. These molecules are expressed as integral membrane proteins of the lipid bilayer of secretory vesicles in neuronal and endocrine cells. An electrochemical gradient provides energy for the vesicular packaging of monoamines, such as dopamine, for later discharge into the synaptic space (reviewed by (Eiden, et al. 2004)). Both immunohistochemistry and gene expression studies show that islet tissue and the beta cells of the endocrine pancreas selectively express only one member of the family of vesicular amine transporters, vesicular monoamine transporter type 2 (VMAT2) (Anlauf, et al. 2003).
VMAT2 is one member of the vesicular transporter family responsible for the uptake and secretion of monoamine neurotransmitters in neurons and endocrine cells. (Zheng et al, 2006) Recent studies have shown the feasibility of noninvasive measurements of the amount of VMAT2 in the pancreas as a useful biomarker of beta cell mass both in humans (R. Goland, et al. 2009) and rodents (Souza, et al. 2006) using [11C] dihydrotetrabenazine (DTBZ) and positron emission tomography, but the possible functional role of VMAT2, as expressed in islet tissue and beta cells, in glucose metabolism has not yet been explored.
As indicated, endogenously synthesized and/or stored monoamine neurotransmitters appear to participate in paracrine regulation of insulin secretion and entrainment of the activity of various cells within islets (Borelli and Gagliardino 2001). Given the important role of vesicular amine transporters in the storage and distribution of monoamine neurotransmitters, the possible of role of VMAT2 in glucose-stimulated insulin secretion was explored using the VMAT2-specific antagonist, tetrabenazine (TBZ) (Scherman, et al., 1983). TBZ acts as a reversible inhibitor of monoamine uptake into granular vesicles of presynaptic neurons (Pettibone, et al. 1984) through its ability to bind to VMAT2 (Scherman 1986) thereby facilitating monoamine degradation by MAO. Monoamine neurotransmitters that are depleted via VMAT2 inhibition by TBZ include serotonin, dopamine, and norepinephrine. Administration of TBZ to rats (plasma elimination with half life, t1/2, equals 2 hours) reduces dopamine levels by 40%, serotonin by 44%, and norepinephrine by 41% in the brain (Lane, et al. 1976).
Although there are other vesicular amine transporters (e.g. vesicular monoamine transporter type 1, or VMAT1), tetrabenazine is highly specific for VMAT2, binds to the transporter with a dissociation constant in the nanomolar range, and displays a more than 10,000-fold reduced affinity towards VMAT1 (Erickson, et al. 1996; Varoqui and Erickson, 1997). Given the known effects of monoamine neurotransmitters on insulin secretion, the expression of VMAT2 by beta cells and the antagonist action of TBZ on monoamine transport, it would be desirable to identify compounds, pharmaceutical compositions and methods to modulate glucose metabolism and insulin secretion to, e.g., prevent, treat, or ameliorate the effects of impaired glucose metabolism, such as, e.g., diabetes and hyperglycemia.